There are two uses for the Metal Coat in Pokémon Brilliant Diamond and Shining Pearl, and arguably the most important of them is evolving certain Pokémon.
The Metal Coat can be held by any Pokémon, and its holder will receive a boost in power to any Steel-type move it uses. This is useful for a Pokémon going against members of the Elite Four, where the boost in the Steel-type move might make the difference between victory and defeat.
In addition to it being a useful in-game item, it’s also used to evolve traded Pokémon. The Metal Coat is used to evolve Scyther into Scizor and Onyx into Steelix. The Scyther or Onyx must be holding the Metal Coat in order for it to evolve once traded. The Pokémon will evolve as soon as the recipient gets it after the trade.
There are several locations trainers can find the Metal Coat in Brilliant Diamond and Shining Pearl, and they can also find a copy of the item by capturing a Magnemite that is holding one. Additionally, the Metal Coat is obtainable after your encounter with Riley on Iron Island.
If you have gotten the Riolu egg from Riley, you can get Metal Coat from Riley’s house, which is the only house next to the boat to leave the Island. There, you can talk to Byron and he will give you the Metal Coat, though it will only be available after you obtain the National PokéDex. It is also unlocked in the Battle Tower, but purchasing one costs Battle Points.
Scyther is exclusive to Brilliant Diamond. The Pokémon appears on Route 229 and in the Grand Underground in that version. Onyx is available in both games in the Oreburgh Mine, Iron Island, Snowpoint Temple, Victory Road, Stark Mountain, and the Grand Underground.